ci: send slack message when ci fails on main (#60)

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<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> Adds a reusable Slack notification workflow and hooks it into the main branch pipeline to alert when the test job fails.
> 
> - **CI/CD**:
>   - **New reusable workflow** `./.github/workflows/notify-slack.yaml` to send Slack alerts via incoming webhook on CI failure, parameterized by `context_title` and `failed_job_name`.
>   - **Main pipeline update** in `./.github/workflows/on-merge.yaml`:
>     - Adds `notify-slack` job that runs after `test` and triggers only on failure, passing context for "main branch" and the failed job name.
> 
> <sup>Written by [Cursor Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit 193fe34. This will update automatically on new commits. Configure [here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

Author: arad-starkware

.github/workflows/notify-slack.yaml

@@ -0,0 +1,51 @@
+name: Notify Slack on Failure
+
+on:
+  workflow_call:
+    inputs:
+      context_title:
+        required: true
+        type: string
+        description: The context title (e.g., "PR #123: Title" or "main branch")
+      failed_job_name:
+        required: true
+        type: string
+        description: The name of the failed job
+    secrets:
+      SLACK_WEBHOOK_URL:
+        required: true
+
+jobs:
+  notify-slack:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Send Slack Notification
+        uses: slackapi/slack-github-action@v2.1.1
+        with:
+          webhook: ${{ secrets.SLACK_WEBHOOK_URL }}
+          webhook-type: incoming-webhook
+
+          payload: |
+            text: "@channel CI failed on ${{ inputs.context_title }}"
+            blocks:
+              - type: "section"
+                text:
+                  type: "mrkdwn"
+                  text: "<!channel> :rotating_light: *CI FAILED on ${{ inputs.context_title }}*"
+
+              - type: "section"
+                fields:
+                  - type: "mrkdwn"
+                    text: "*Workflow:*\n`${{ github.workflow }}`"
+                  - type: "mrkdwn"
+                    text: "*Failed Job:*\n`${{ inputs.failed_job_name }}`"
+                  - type: "mrkdwn"
+                    text: "*Commit:*\n`${{ github.sha }}`"
+                  - type: "mrkdwn"
+                    text: "*Author:*\n`${{ github.actor }}`"
+
+              - type: "section"
+                text:
+                  type: "mrkdwn"
+                  text: "<https://github.com/${{ github.repository }}/actions/runs/${{ github.run_id }}|View failed job run>"
+

.github/workflows/on-merge.yaml

@@ -9,3 +9,12 @@ jobs:
   test:
     uses: ./.github/workflows/test.yaml
     secrets: inherit
+
+  notify-slack:
+    needs: test
+    if: failure()
+    uses: ./.github/workflows/notify-slack.yaml
+    with:
+      context_title: "main branch"
+      failed_job_name: "test"
+    secrets: inherit